In response to an increasing threat of terrorism, inspection of persons and other transportable items for weapons and other types of contraband is becoming essential at security checkpoints, such as those found at airports, concerts, sporting events, courtrooms, federal buildings, schools and other types of public and private facilities potentially at risk from terrorist attacks. Conventional security inspection systems currently in place at security checkpoints include physical inspection, such as visual and/or tactile inspection, performed by security personnel, metal detectors and X-ray systems. However, physical inspection by security personnel is tedious, unreliable and invasive. In addition, metal detectors are prone to false alarms, and are not capable of detecting non-metallic objects, such as plastic or liquid explosives, plastic or ceramic handguns or knives and drugs. Furthermore, X-ray systems pose a health risk, particularly to those people who are repeatedly exposed to X-ray radiation, such as airport personnel, and X-ray systems are also not able detect certain materials/geometries, such as ceramic knives.
As a result of the need for improved security inspection systems, various microwave imaging systems have been proposed as an alternative to existing systems. Microwave radiation is generally defined as electromagnetic radiation having wavelengths between radio waves and infrared waves. An advantage of microwave radiation over X-ray radiation is that microwave radiation is non-ionizing, and therefore, poses no known health risks to people at moderate power levels. In addition, over the spectral band of microwave radiation, most dielectric materials, such as cardboard, plastic cloth, leather and dry wood are nearly transparent. Therefore, microwave imaging systems have the ability to penetrate wooden or plastic crates, luggage bags, envelopes, cloth bags and other types of transportable items.
At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors to capture either passive microwave energy emitted by the target or reflected microwave energy reflected from the target in response to active microwave radiation of the target. A two-dimensional or three-dimensional image of a person or other item is constructed by scanning (moving) the array of detectors with respect to the item's position and/or adjusting the frequency (or wavelength) of the microwave energy being transmitted or detected. For example, an article by David M. Sheen et al., “Three-Dimensional Millimeter-Wave Imaging for Concealed Weapon Detection,” IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No. 9, September 2001, pp. 1581–1592 describes a three-dimensional holographic microwave imaging technique that uses a scanning bar to mechanically move a linear array of detectors to scan an item or person. The resulting measured data is used to reconstruct a holographic image of the item. However, such scanning systems typically require mechanically moving parts and/or intensive post-processing reconstruction of the image, both of which increase the cost and complexity of the microwave imaging system.
Another technique uses lenses to focus a beam of microwave radiation on the array of microwave detectors. This type of technique is described, for example, in the article P. F. Goldsmith, et al., “Focal Plane Imaging Systems for Millimeter Wavelengths,” IEEE Transactions on Microwave Theory and Techniques, Vol. 41, No. 10, October 1993, pp. 1664–1675. However, microwave imaging systems that use lenses to focus the microwave energy typically have a limited field of view and small aperture size. In addition, the cost of the lens system may be prohibitively large in many applications.
Therefore, what is needed is a cost-effective and simple microwave imaging system for use in security inspection of transportable items that does not require any moving parts.